The Northeast Cold Vortex At Different Latitudes Has Significantly Different Climate Impact

The active Northeast China cold vortex(NCCV)in early summer not only bring local effects on precipitation and temperature over northeast China,but also exert non-negligible climatic effects on precipitation in the middle and lower reaches of Yangtze River in China.In this work,the NCEP/NCAR daily mean and monthly mean reanalysis data are used to identify the May-June NCCVs in different positions during 1981-2020 via using the NCCV automatic identification program to reveal their frequency and the existence of different NCCV positions modes.The multivariate empirical orthogonal function(MV-EOF)method is conducted on June 500 h Pa geopotential height and air temperature over the active region of the NCCV to explore the distinct climate effects of NCCV modes in different positions and finally,possible external forcing factors modulating the NCCV modes are discussed.The main results are as follows.(1)The NCCV are active in early summer especially in June,the northern NCCV and the middle ones are more frequent than the Southern NCCV.The frequency of NCCV contributes to the formation of NCCV modes in different positions.After verification,two leading modes are identified.The second leading mode effectively characterizes the different positions of the NCCV while the first leading mode represents the NCCV intensity on monthly time scale.When it is in positive phase,the NCCV are active around south of 50°N,which is named as the southward NCCV mode(SCVM),when it is in negative phase,the NCCV are active in the north of 50°N,which is called the northward NCCV mode(NCVM).(2)The different NCCV modes exert different climate effects on summer precipitation over eastern China.For the NCVM,the pressure gradient between the lower reaches of the Yangtze River and Japan is weakened,and the northerly winds dominate the middle and lower reaches of the Yangtze River and Japan and Korea,resulting in a decreased precipitation from the middle and lower reaches of the Yangtze River to Japan.On the contrary,for the SCVM,the pressure gradient between the NCCV active area and the northwest Pacific subtropical high(WPSH)active area is enhanced,which is conducive to the downward momentum transformation.The water vapor from the South China Sea is transported to the middle and lower reaches of the Yangtze River,and the middle and lower reaches of the Yangtze River have better rising conditions,resulting in a significant increase of precipitation in the middle and lower reaches of the Yangtze River at this time.It can be concluded that the climate effect of the NCCV on precipitation is asymmetric,and this climate effect can last until July.In particular,the extreme Meiyu event in the summer of2020 is consistent with the SCVM climate effect obtained in this paper.(3)Considering the climate effects of the NCCV in different positions will effectively improve the prediction accuracy of summer precipitation over eastern China.Using the two time series of MV-EOF as the intensity index and location index of the NCCV mode to reconstruct precipitation respectively,For SCVM,approximately 50% of the precipitation could be forecasted in the middle and lower reaches of the Yangtze River with an increase of10%(32%)in June(July)compared with that without NCCV position.For NCVM,the forecasted precipitation could explain 65%(36%)variance of the observation in June(July)over the Beijing-Tianjin-Hebei region.(4)The possible external forcing factors affecting the north-south position of the NCCV are complex.Further analysis shows that the NCVM is intimately associated with central Pacific El Ni?o events while the SCVM may be modulated by the abnormally reduced sea ice melt amount from spring to summer.